Such an apparatus includes: at least one mechanically oscillatable unit, wherein the mechanically oscillatable unit is an oscillatory fork, a single rod, or a membrane oscillator; at least one exciting/receiving unit, which excites the mechanically oscillatable unit, such that it executes mechanical oscillations, and which receives mechanical oscillations of the mechanically oscillatable unit; and at least one electronics unit, which supplies the exciting/receiving unit with an electrical, exciter signal SE, also referred to herein as output signal SE, and which receives from the exciting/receiving unit an electrical, received signal SR, also referred to herein as input signal SR. Provided in the electronics unit are at least one input amplifier and at least one output amplifier. The electronics unit exhibits a total amplification factor. The electronics unit serves especially for feedback. The medium is, for instance, a liquid or a bulk good. The process variable is, for example, fill level, density or viscosity of the medium.
In the state of the art, measuring devices are known, in which a so-called oscillatory fork, as a mechanically oscillatable unit, is excited to execute oscillations. Since the oscillations, i.e. their characterizing variables, such as frequency, amplitude and phase, depend on whether there is contact with a medium, and then on its properties, such as density or viscosity, such variables can be deduced from the characterizing variables of the oscillations. Thus, such a measuring device makes possible, for instance, the monitoring of fill level or the measuring of density of the medium.
A difficulty is that the amplitude of the input signals SR of the electronics unit, thus the received signals from the mechanically oscillatable unit, can differ widely. For instance, if the oscillatable unit is oscillating not covered by the medium, then a large amplitude results, while, when the oscillatable unit is covered by the medium, then the amplitude decreases, on occasion, considerably. Such behavior is associated in the electronics with problems such as amplitude-dependent phase shifting, non-linearity, signal-noise separation, and/or saturation. Furthermore, the amplifier has to be correspondingly designed, in order to be able to cover the resulting dynamic range.